A steam turbine is understood in the context of the present application to mean any turbine or subturbine through which a working medium in the form of steam flows. In contrast to this, the working medium flowing through gas turbines is gas and/or air which, however, is subject to completely different temperature and pressure conditions from the steam in a steam turbine. In contrast to gas turbines, in steam turbines, for example, the working medium flowing into a subturbine has the highest temperature and at the same time the highest pressure.
A steam turbine conventionally comprises a rotatably mounted turbine shaft which is equipped with blades and which is arranged within a casing jacket. When heated and pressurized steam flows through the flow space interior formed by the casing jacket, the turbine shaft is set in rotation via the blade by the steam. The blades of the turbine shaft are also designated as moving blades. Furthermore, stationary guide vanes are suspended on the casing jacket in a conventional way and engage into the interspaces of the moving blades. A guide vane is conventionally held at a first point along an inside of the steam turbine casing. It is in this case conventionally part of a guide vane ring comprising a number of guide vanes which are arranged
along an inner circumference on the inside of the steam turbine casing. Each guide vane in this case points with its blade leaf radially inward.
Steam turbines or steam subturbines may be divided into high-pressure, medium-pressure or low-pressure subturbines. Where high-pressure subturbines are concerned, the inlet temperatures and inlet pressures may amount to a maximum of 700° C. and 300 bar respectively, depending on the material used. A sharp separation between high-pressure, medium-pressure or low-pressure subturbines has hitherto not been defined uniformly among experts.
According to DIN standard 4304, a medium-pressure subturbine is obtained when this medium-pressure subturbine is preceded by a high-pressure subturbine into which fresh steam flows, and when the outflowing steam from the high-pressure subturbine is intermediately superheated in an intermediate superheater and flows into the medium-pressure subturbine. According to the standard DIN 4304, a low-pressure subturbine is defined as a turbine which receives the expanded steam from a medium-pressure subturbine as fresh steam.
Single-casing steam turbines are known which constitute a combination of a high-pressure and of a medium-pressure steam turbine. These steam turbines are characterized by a common casing and a common turbine shaft and are also designated as compact subturbines.
Compact subturbines are designed with forms of construction which are designated by reverse-flow or by straight-flow. In the straight-flow form of construction, the fresh steam flows into the steam turbine and spreads essentially in the axial direction of the steam turbine through the high-pressure subturbine, is then recirculated to the intermediate superheater unit into the boiler and passes from there into the medium-pressure subturbine.
In the reverse-flow form of construction, the fresh steam flows through the outer casing and there impinges essentially onto the middle of the turbine shaft and subsequently flows through the high-pressure subturbine. The expanded steam flowing out downstream of the high-pressure subturbine is intermediately superheated in an intermediate superheater and flows into the steam turbine again at a suitable point upstream of the medium-pressure subturbine. The flow directions of the steam in the high-pressure subturbine and in the medium-pressure subturbine are in this case opposite to one another.
The turbine shaft must meet particular requirements on account of the various temperatures of the steam. Heat-resistant properties are demanded in the inflow region of the high-pressure subturbine. High long-time rupture strengths under centrifugal force are required at the ends of the turbine shaft. Furthermore, good toughness properties and tensile strengths are desired.
Monobloc turbine shafts consisting of one material have been used hitherto in compact subturbines. Particularly for high power outputs, the production of these monobloc turbine shafts signifies a costly solution. A further disadvantage of these monobloc turbine shafts is that relatively costly build-up welds have to be applied at the bearing points.